There are a wide variety of ways of storing data persistently, particularly with cloud-based systems. These include file systems, relational databases (e.g. DB2, MySQL, SQL Server), NoSQL systems, and the like.
The emergence and popularity of in-memory NoSQL databases (often interpreted as “not only SQL” where SQL refers to structured query language) can be attributed to the flexible data model and the huge performance gain they provide as compared with a traditional relational database management system (RDBMS). In particular, NoSQL databases adopt flexible, schema-less data models, which eases application usage and fits well to the need of many applications. In addition, by relaxing the stringent design properties required by traditional RDBMS, NoSQL databases can often benefit from a less sophisticated design architecture, which yields much greater performance as the database scales. As a result, open-source and proprietary NoSQL data stores, such as, for example, Memcached, Redis, Voldemort, MongoDB and Couchbase, have gained huge grounds in terms of market adoption in recent years.